1. Field of the Invention
The present invention relates to an internal combustion engine lubrication system in which maximum pressure within the lubrication system is controlled by a dual mode pressure limiting valve.
2. Related Art
Pressure-lubrication has been used with automotive internal combustion engines for many years. Typically, a single pressure relief valve is used to limit the maximum pressure developed by an oil pump. The relief pressure is usually set at a fairly high value, in order to assure that the engine receives adequate lubrication in all operating regimes from idle to maximum speed-load operation. Unfortunately, this causes the lubrication pump's energy consumption to be higher than would otherwise be ideal, because in many operating regimes, only a reduced volume and pressure of oil is required by the engine.
US Patent Publication 2002/0083915 discloses a system in which a minimum oil pressure is established by a mechanically operated valve, with an electrically operated valve positioned in the mechanical valve's relief port such that the system pressure may be increased through closure of the electronically operated valve. This system suffers from the drawback that the electrically operated valve does not have a fail-safe position. In the event that the electrically operated valve fails in an open position, only minimal oil pressure would be available to sustain the engine, and such minimal oil pressure would undoubtedly be inadequate to support the engine during high speed and load operation. On the other hand, if the electrically operated valve fails in a closed position, the resulting excessive oil pressure may damage or destroy the engine by causing the oil filter to rupture, with a consequent loss of all oil overboard. In other words, any failure of the electronic valve could lead to engine failure. In another vein, the reliability of the electrically operated valve is compromised because only unfiltered oil flowing from the oil pump passes through the valve, and foreign matter carried by the unfiltered oil may cause the valve to stick or wear excessively.
It would be desirable to provide an internal combustion engine lubrication system in which an engine is reliably protected with a redundant high pressure mechanical relief valve, combined with the capability of reducing the oil pressure so as to promote fuel economy improvement during operating regimes in which maximum oil pressure and flow are unneeded.